As the growing season gears up, it is time to consider fertilizing garden plants. But figuring out how and when to responsibly use fertilizers can be confusing and improper fertilization can be harmful to plants and to the environment. Below are a few commonly asked questions about fertilizers.
What do the numbers on a fertilizer bag represent?
The numbers indicate the amounts or percentages of nitrogen, phosphate, and potash in the fertilizer. These three nutrients are sometimes limiting to growth and health of plants in the garden. The first number refers to the amount of nitrogen, the second refers to the amount of phosphate, and the third refers to the amount of potash. For example, a 10-6-4 contains approximately 10% nitrogen, 6% phosphate, and 4% potash.
There are so many fertilizers out there, which one is best for my plants?
Deciding what type of fertilizer to use can be a bit confusing. Look at the fertilizer label first. Fertilizer labels often tell you the type of plant it was formulated for – for example if it is an orchid fertilizer this will often be prominent on the label. Lawn fertilizers generally contain high levels of nitrogen (first number) to promote vegetative or leafy growth. Lawn fertilizers are great for your grass, but lousy for annual flowers (too much nitrogen promotes excessive vegetative growth and inhibits flowering). General purpose fertilizers are often complete (contain nitrogen, phosphate, and potash) and are good for a wide range of plants in the garden, landscape, or containers. In the label directions, you will often get information on different application rates for different groups of plants. If you want to fertilize houseplants and an application rate is not recommended for houseplants on the label, you may want to consider another fertilizer.
Are organic fertilizers better than synthetic fertilizers?
From a plant perspective, both sources can supply the nutrients needed. There are pros and cons with both types of fertilizers. Organic fertilizers are often lower in analysis (less nitrogen, phosphorus, potassium, etc.) so they are less likely to cause injury due to over application. Yet, this lower analysis may mean you may need to apply more product in poor soils. Organic fertilizers may not supply specific nutrients, while synthetic formulations can include many macro- and micro-nutrients. However, organic fertilizers, like composted manures, can also improve soil quality over time, unlike synthetic fertilizers. Many gardeners will use both organic and synthetic fertilizers, depending on the situation. For example, some gardeners will add composted manures to the vegetable garden in fall or early spring to improve soils and then still add granular synthetic fertilizers as a top dressing after planting for long-season vegetables like tomatoes.
What form of fertilizer should I use?
Home gardeners commonly use granular or liquid forms of fertilizer. Granular fertilizers can be water soluble (fast dissolving) or slow-release materials. Slow release fertilizers are formulated to release nutrients slowly over several weeks or months, so one application in spring may be all that is necessary. Liquid fertilizers are fast-acting and can be added to the water you use to irrigate your plants.
How much and how often should I fertilize my plants?
The rate and frequency of application depends on the nutrient analysis of the fertilizer, plant species, soil type, and other environmental factors. For outdoor or garden plants always start with a soil test to determine the amounts of fertilizer needed. There are some general guidelines on how often to fertilize plants. A granular fertilizer is often applied to vegetable gardens at the beginning of the growing season, usually at planting or prior to planting. Annual flowers may require frequent fertilization throughout the growing season, especially if they are growing in containers. Houseplants need regular fertilization in spring, summer, and fall. Many houseplants do not need fertilizer in the winter. Established perennials may need fertilizer once in the spring every other year. Established trees and shrubs rarely need fertilizer. But these are all general recommendations and could vary, so watch your plants. Poor or slow growth and overall yellowing are signs that a plant may be lacking essential plant nutrients and would benefit from an application of fertilizer.
Can you fertilize too much?
Yes, over-fertilization can burn plant leaves, stunt growth, and create environmental issues. More is rarely better when using fertilizers. Always read and follow label recommendations for fertilizer rates, dilutions, and application guidelines. If you are ever in doubt, always err on the conservative side or use less fertilizer.
My plant looks sick. Should I fertilize it?
First you must figure out why the plant is not doing well. Ask yourself the following questions: “Has it been over-watered or under-watered? Is it receiving enough light or too little light? Are there signs of disease or other pests?” before pulling out the fertilizer bag. Fertilizer doesn’t fix all problems and in some situations, it can make the problem worse.
Why are some fertilizers blue or pink?
Dyes (usually blue) are added to some fertilizers to identify them as a fertilizer and so that you can loosely determine how much you are applying. As you would expect, a dark blue solution contains higher amounts of nutrients than a light blue solution.
Are fertilizers harmful to pets or kids?
Fertilizers, like many other household chemicals, should be kept out of the reach of children and pets.
Hopefully these questions and answers will help you make good decisions on using fertilizers in your garden or landscape this year.
Some additional tips on fertilizers.
- ALWAYS read and follow label directions before applying any fertilizer.
- Avoid applying fertilizers when the soil is dry. (This increases the chances for burning the foliage.)
- Newly planted trees, shrubs, and perennials rarely need fertilizer during the first growing season. (Regular applications of water, to aid in root establishment, are more important.)
- For container plants in soilless media:
Regular fertilizer is important when the plant is growing.
If you use liquid or water soluble fertilizers, avoid fertilizing every time you water.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Horticulture and Home Pest News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on May 21, 2021. The information contained within may not be the most current and accurate depending on when it is accessed.